Among portable devices such as portable game devices, portable terminals, and the like, proposed is a portable device with a rear surface input system wherein an operational touch panel is set on the rear surface of the portable device, and a user can perform an input operation with a finger on the rear surface (see Japanese Laid-open Patent Publication No. 2000-278391, Japanese Patent No. 4569411, Japanese Patent No. 4587616). When adopting such rear surface input system, since it does not happen that a part of the display screen is covered by a finger in operation, the visibility of the display screen can be improved.
However, conventional portable devices with the rear surface input system have a problem in their operability because a user's finger is placed around the rear surface. That is, the user has difficulties in making the finger touch the intended position (coordinate) precisely on the rear surface of the device, and in making accurate input operations.
In order to improve the operability of the rear surface input, could be contrived such configurations as the rear surface input part configured with a transparent material, so that the user's finger placed around the rear surface of the device is seen through; with a sensor other than a touch panel, such as a camera, used in combination, so that the imaging of the finger is presented on the screen; or with a protruding object added onto the rear surface of the device, so that the tactile sense of the protrusion enables the user's finger to feel for the input position and to operate. However, such configurations would necessitate other technologies than the technology of a regular touch panel and would complicate the device. Moreover, the transparency of the device and the installation of a sensor would restrict the designing of the device, and since the foregoing configurations are realized by different technologies from hardware to hardware with no technological versatility, the manufacturing cost would increase.
Moreover, with a portable device, where operations are performed on the rear surface as the device is held by the user's hands, by jiggling or the like, an erroneous operation is likely to occur, such as pushing a wrong button. Specifically, a portable device can be used in cars or during walking; when a portable device is used under these circumstances which easily cause jiggling and the like, even if the transparency of the device or else is accommodated, an accurate input operation onto the rear surface would be difficult.